To complement biochemical and fluorescence light microscopy studies on papillomavirus infection and virion assembly being conducted by NCI investigators, we are determining the high-resolution distribution of the papillomavirus capsid protein L2 within infected epithelial cells grown in culture. Infected cells were pelleted, high-pressure frozen, freeze-substituted, plastic embedded and sectioned. Some sections were stained and imaged at a beam voltage of 120 kV in an energy-filtering transmission electron microscope to characterize the cellular ultrastructure, while other sections were immunolabeled with gold-conjugated antibodies against the L2 virion protein. Experiments are in progress to improve specificity of the immunolabeling in order to localize the capsid proteins within the infected cells, particularly in specific regions of the nucleus.